In fuel-injection type internal combustion engines, it is known to employ an open loop type control system which electronically controls the combustion-determination variables such as ignition timing, fuel amount for each combustion, fuel injection timing and exhaust gas amount recirculated such that these variables automatically but independently take their predetermined values in accordance with various operation modes of the engine.
However, in fact, their predetermined values of the combustion-determination variables are not chosen from values which are most suitable for operating the engine effectively and economically. This is because, for safety operation of engine, the determination of such values is made by considering the unavoidable dimentional errors in manufacturing the engine proper, the fuel supply system and spark ignition system, the temperature and pressure changes in atmosphere, and the property variations of fuels to be used.
Apart from the above, it is also known to use for electronically controlling the engine a closed loop type or feedback-type control system which electronically and totally controls the combustion-determination variables such as ignition timing and fuel injection timing in accordance with information signals issued from various sensors which detect the operation condition of the engine.
One of the feedback-type control systems is such constructed that for avoiding knocking phenomenon, the ignition timing is automatically advanced or delayed to be converged to its optimal timing value as much as possible.
In this feedback control system with the knocking avoiding means, a vibration sensor having a bandpass filter is mounted on the engine proper for selectively sensing specific frequency vibration of the engine which is exclusively generated when the knocking occurs. By computing or processing the information signals from the vibration sensor to monitor the degree of knocking condition, the ignition timing is automatically converged to its optimal timing position. (In practical use, however, the setting of the ignition timing is so made as to allow occurrence of slight knocking since the power generating efficiency and fuel consumption of the engine show their optimal values when the ignition timing is set at a position where slight knocking occurs.)
However, in this type feedback control system, a problem arises inevitably because of its inherent arrangement mentioned above. Even under operation without generation of knocking, the engine runs with vibration which contains a considerable amount of components of which frequency matches the specific vibration frequency exclusively generated upon knocking. Thus, higher detecting ability to knocking is not expected from such arrangement, so that accurate feedback control for achieving optimal ignition timing is not achieved.